How does sole proprietor calculate PAYROLL for PPP

The client is a sole proprietor and independent contractor who gets paid on a 1099 basis and has no employees of his own. He deposits the income checks into his personal checking account and uses that account to pay both his personal bills as well as his business bills such as business expenses and utilities as well as mortgage payments. Under PPP loan application, the loan is based on PAYROLL amount. He does not write a check to his own name. He files taxes with a Schedule C. How does he calculate PAYROLL?